Many biological functions are maintained with a periodicity of approximately 24 hours and are therefore termed circadian. The role of the suprachiasmatic nuclei (SCN) in generating circadian rhythms in behaviors, endocrine functions, and other physiological processes is well established. Approximately 500 neurons immunoreactive for calbindin (CB) in the caudal .region of the hamster SCN have been identified as critical in SCN grafts for the restoration of circadian locomotion in SCN- lesioned hamsters. Calbindin-D28K, a calcium-binding protein found in select neuronal populations within the brain, binds calcium selectively and with high affinity. This proposal aims at investigating characteristics of this population of SCN neurons that has retinal input and circadian output functions. Using electrophysiological and immunohistochemical techniques, I will determine if action potential firing patterns of CB- immunoreactive neurons are circadian. Second, I will investigate how CB-immunoreactive (Cbir) neurons respond to vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP), neuropeptides localized in the same region of the SCN. Third, I will determine how VIP modulates the synaptic stimulation of CBir neurons.